Photographic film is used in the process of creating image reproducing materials. Included within the definition of image reproducing materials are materials made in contact mode including printing plates, proofs (proofing materials), films and resists, including printed circuit board resists.
The preparation of printing plates or proofs utilized in the newspaper printing industry involves several steps. A latent image representing a computer based image is placed on a photographic film. The latent image in the film is then developed. The developed film is then placed next to a photographic plate, or proof, and exposed to ultraviolet light. The ultraviolet light creates a pattern in the photographic plate corresponding to the developed image. The plate can then be inked and used to print the newspaper copy. Correspondingly, the proof created from the developed film can be viewed.
Creation of the latent image on a photographic film typically uses computer driven imagesetting equipment. Such imagesetting equipment, well known in the art, uses a computer based image to drive a radiation generator, generally a laser, laser diode or cathode ray tube, which is scanned across radiation sensitive film. Modulation of the radiation based upon the computer based image creates a corresponding image on the radiation sensitive film.
In this industry and others, imaging materials sensitized to the red may be used to produce hard copy from computer stored graphics data using a scanning laser diode source. In particular, halftone graphic art contact films can be generated which can be used in the color reproduction process to image ultraviolet photosensitive materials, such as printing plates or proofs and color proofing films. There is a need for photographic elements containing higher concentrations of antihalation and acutance dyes which absorb in the red region of the electromagnetic spectrum with little or no absorption in the ultraviolet region of the electromagnetic spectrum.
The silver halide film utilized in this process is traditionally wet chemistry photographic film. Such wet chemistry film provides the necessary acuity and density. However, the use of wet chemistry in the development of the image incurs a substantial environmental impact, a significant cost of and in maintaining the chemistry and involves a substantial time impact in the process.
The increasing availability and/or use of higher output gas lasers or light emitting diodes emitting in the red region of the electromagnetic spectrum has created the opportunity to use photothermographic imaging materials in place of conventional silver halide materials which are sensitive to this region. Due to environmental concerns, it is desirable to replace conventional silver halide materials which are processed by known `wet` chemistries with `dry` processed materials. Wet processing produces waste materials which are becoming more difficult to dispose of, both commercially and environmentally. Dry processed materials do not suffer from these disposal considerations.
Photothermographic film is also well known in the art. U.S. Pat. No. 3,457,075, Morgan et al, Sensitized Sheet Containing an Organic Silver Salt, a Reducing Agent and a Catalytic Proportion of Silver Halide, assigned to Minnesota Mining and Manufacturing Company, the assignee of the present invention, describes a light sensitive film for dry photography which form high contrast images when exposed to a light image and then uniformly moderately heated. The sensitized sheet described in Morgan et al is commonly known as a photothermographic film or "dry silver" film.
Photothermographic imaging systems are those imaging materials which, upon first being exposed to light in an image-wise fashion, produce an image when subsequently heated, The exposure to light or other radiation photoactivates or photodeactivates a component in the imageable element and subsequent heating causes an image forming reaction to differentially occur in exposed and unexposed regions. It is because the exposure and development of the imaging systems occur without using water, that these materials are often referred to as dry silver materials.
Such dry silver film, however, is not suitable for use with the above described process for producing newspaper printing plates or proofs. Typically, either the laser power is too low, the film speed too slow, or both.
If a higher power laser is utilized in the process, light scattering, or halation, becomes a significant problem. This produces loss in exposure latitude making it difficult to achieve acceptable performance in a consistent manner.
Light sensitive recording materials may suffer from a phenomenon known as halation which causes degradation in the quality of the recorded image. Such light which strikes the photosensitive layer is not absorbed but passes through to the film base on which the photosensitive layer is coated. A portion of the light reaching the base may be reflected back to strike the photosensitive layer from the underside. Light thus reflected may, in some cases, contribute significantly to the total exposure of the photosensitive layer. Any particulate matter in the photosensitive element may cause light passing through the element to be scattered. Scattered light which is reflected from the film base will, on its second passage through the photosensitive layer, cause exposure over an area adjacent to the point of intended exposure. It is this effect which leads to image degradation. Photothermographic materials are prone to this form of image degradation since the photosensitive layers contain light scattering particles. The effect of light scatter on image quality is well documented, and is described, for example, in T. H. James "The Theory of the Photographic Process", 4th Edition, Chapter 20, Macmillan 1977.
In order to improve the sharpness or definition of photographic images an antihalation layer is often incorporated into photosensitive compositions. To be effective, the active ingredient in the antihalation layer will absorb at the wavelengths oat which the photosensitive composition is sensitive. The longer the path length of the light in the layer of light-sensitive composition, the greater the attenuation. Therefore, scattered light is attenuated or absorbed to a larger extent than light which impinges directly on a light-sensitive crystal. As a result, although the overall speed of the composition is reduced slightly, scattered light and other light rays which are liable to produce a blurred image are preferentially absorbed and so the overall definition and sharpness of images produced in the layer are increased.
Antihalation compounds, known in the art as acutance agents, are dyes or pigments that can be incorporated into photosensitive systems. Preferably they are heat lablie in the system, that is to say, they are degraded by the heat development of the photothermographic composition to one or more compounds which are substantially colorless. The exact mechanism of this reaction is not known. Such acutance agents are disclosed in, for example, U.S. Pat. No. 4,308,379.
United Kingdom Patent No. 1,261,102, Minnesota Mining and Manufacturing Company, Transparent Heat-Developable Photosensitive Sheet Material, describes the use of transparent heat-developable light sensitive materials. The "image sharpness or acutance of these "dry silver" transparencies [is improved] by incorporating in the sensitive layer small proportions of light-absorptive colouring matter" (Page 1, lines 41-44). The materials described in the UK '102 patent specifically requires that "the amount of coloured material added directly to the silver salt composition must be kept small since the color necessarily will remain in the image layer" (Page 1, lines 53-57).
The photosensitive sheet material described in the UK '102 is not suitable for use in the plate producing process described above. Imagesetting equipment used to produce film for this process have a power, typically 1 milliwatt, due to the necessary speed of travel of the laser in the imagesetter and the requirement for optical densities of 2.5 or greater. At this power level, the halation problem is not resolved. The photosensitive material described in the UK '102 patent creates unacceptable dot gain and unacceptable edge acuity.
Alternatively, antihalation compounds can be included in a layer separate from the radiation-sensitive layer. U.S. Pat. No. 4,477,562, Zeller-Pendrey, Dry Strippable Antihalation Layer for Photothermographic Film, assigned to Minnesota Mining and Manufacturing Company, the assignee of the present invention, describes a strippable antihalation layer which can be used with a photothermographic film. The antihalation layer is placed in a separate strippable layer because, in part, it is advantageous so as to avoid stain in the imaged area (Col. 2, lines 45-48). Similarly the aforementioned UK '102 patent also describes the use of a large amount of antihalation material in a separate strippable layer. The "image sharpness or acutance of these "dry silver" transparencies [is improved] by incorporating relatively large proportions of such coloured materials in separate layers which may subsequently be removed by a dry stripping process" (Page 1, lines 41-50). The materials described in the UK '102 patent specifically requires that "very much larger quantities of coloured material may be employed in coatings provided they are subsequently to be removed" (Page 1, lines 80-83).
The photosensitive material described in both of these documents locate the large amount of antihalation material in a separate layer which must be removed from the film before the film is used.
Similarly, U.S. Pat. No. 3,769,019, Wiese Jr., et al, Light and Heat Sensitive Sheet Material, describes a photosensitive sheet material having a first colored heat-bleachable layer to prevent light scattering/reflection back into a second transparent light-sensitive heat-developable "dry silver" layer as the material is exposed to a light image. The sheet is then heated to develop the visible image in the second layer. The heating simultaneously causes discharge of the color in the colored first layer so that non-image areas of the developed print are transparent.
As in the Zeller-Pendrey '562 and in the UK '102 patent, the antihalation material contained in the photosensitive sheet described in the Wiese Jr. et al patent must undergo another processing step to eliminate the colored antihalation material before the photosensitive sheet can be used for further processing steps.
Thus, these prior art photosensitive systems require either that the antihalation material is kept small or require that the antihalation material be contained in a separate layer that is removed from the material or rendered colorless.
U.S. Pat. No. 4,581,325, Kitchen et al, Photographic Elements Incorporating Antihalation and/or Acutance Dyes, and the similar Great Britain Patent Specification No. 89.312472.7, Infrared-Sensitive Photographic Materials Incorporating Antihalation and/or Acutance Dye, describe a conventional wet processed type or dry silver type photographic element which includes an antihalation dye which absorbs in the near infrared (above 700 nanometers) and has a relatively low visible absorption. This material is also unsuitable for use in the above-described process for producing newspaper printing plates or proofs for imagesetting equipment that radiates radiation in the 630-700 nanometer range. Hence, the antihalation material would be ineffective at these wavelengths.